This invention relates to a protection device against flashover in a circuit, in which a first discharge gap is connected in parallel with the circuit to be protected and is constructed to produce an arc when a voltage applied to the circuit exceeds a selected value.
Protection devices of this type are described in DE-OS No. 28 34 029 and DE-PS No. 28 33 992, in which the circuit of a radio transmitter to be protected, e.g. consisting of a switching and output stage tube and a free-running diode, is protected by a discharge gap, e.g. an ignitron, connected in parallel with the circuit. The discharge gap will be fired by an ignition circuit whenever an undesirable overvoltage occurs in the circuit to be protected.
If such a radio transmitter is designed, for example, as a pulse-duration modulated high power transmitter with an output power such as 600 kW, a d.c. voltage of about 30 kV is required at the circuit to be protected. A discharge gap connected in parallel with the circuit points across which the 30 kV voltage is present should possess higher electric strength in the non-conductive state so that no spurious ignition of the discharge gap occurs which causes an undesired disruption.
At present, ignitrons usable for such purposes can only withstand a maximum direct voltage of about 50 kV in the non-conductive state at an ambient temperature of 20.degree. C. Owing to physical properties, this maximum direct voltage drops to about 35 kV at an ambient temperature of about 45.degree. C. Since such ambient temperature will occur if, for example, the transmitter is operated in a tropical climate, a safety margin of about 5 kV with respect to the continuous operation voltage (30 kV) is insufficient.
Small transient overvoltages, which are unavoidable in transmitter operation, would cause disruption by so-called overhead ignition of the ignitron, i.e., an ignition occurring via the anode-cathode gap, thus bypassing the ignition electrode, or igniter.
To prevent such a problem, it would make sense to cool the ignition properly. But, particulary in tropical climates, this requires costly air conditioning. It would also be logical to connect at least two ignitrons in series and to ignite these by means of at least one common ignition circuit. It should be noted, however, that at least one of the ignitrons is set in the non-conductive state at a high voltage. The ignition circuit should therefore offer corresponding electric strength. But such an ignition circuit is not cost-effective.